Capacitance type liquid quantity gauge



1952 s. J. SMITH 2,582,399

CAPACITANCE TYPE LIQUID QUANTITY GAUGE Filed Jan. 12, 1949 the following description of the invention with Patented Jan. 15, 1952 2,582,399 CAPACITANCE -LQUID QUAN Stanley James Smith, Owning, N. Y. Application January 12, 1949, Serial No. 70,494

In capacitance type liquid quantity gauges, measurement of liquid quantity is effected by means of a capacitor which usually comprises two spaced electrodes extending between the top and the bottom of a container for the liquid. For reasons of convenience in manufacture and of structural strength, such measuring capacitors are commonly tubular in form and comprise two aluminium tubes .separated by a minimum of solid dielectric material. Such a construction provides a capacitor in which the capacitance per unit length is substantially constant, and a linear change in capacitance in terms of liquid level will be obtained. I

Since the electrical system of a capacitance type gauge is usually designed so that the indication is proportional to capacitance change it follows that the indicator may be calibrated linearly in terms of liquid level and also in terms of liquid quantity or mass if the container is of regular shape so that the surface area of the liquid is constant at all levels.

However, it is not unusual for liquid containers to be constructed of irregular shape in which the relation between change in liquid quantity and change in level at a given point is not constant. Examples are aircraft fuel tanks which are fitted to the airplane structure and which may assume a variety of relatively complex shapes. In these cases, it is possible to calibrate the dial of the indicating device so that the correct quantity will be indicated. but the scale will be non-linear and difllcult to read. When more than one measuring capacitor is used in a tank, in order to correct for changes in aircraft attitude, it is essential that the change in capacitance per unit volume or mass of fuel be identical for all units and this is also necessary in cases where the contents of two or more tanks must be displayed on one indicator.

It is known for this purpose to modify the normally uniform capacitance-length law of a measuring capacitor consisting of concentric metal tubes by control of the diameter of or removal of wall material from one of the tubes. Such methods however weaken the structure and are not adapted toquantity production of capacitors to close tolerances.

The present invention has for an object the provision of an improved form of measuring capacitor in which the capacitance per unit length may be made to follow any desired law.

Other objects of my invention and the advantages flowing therefrom will be apparent from which is made of an 1. Claim. (Cl. -415) 2 reference to the accompanying drawing, in which Figure l is an elevation of one form of measuring capacitor;

Figure 2 is a vertical section of the capacitor shown in Figure 1;

Figure 8 is a developed view of the surface of the inner electrode of the capacitor of Figures 1 and 2;

Figure 4 is a similar view of an alternative form of inner electrode;

Figure 5 is a cross section through a capacitor using the electrode of Figure 4;

Figure 6 is a view, similar to that of Figure 3, of a further alternative form of inner electrode;

Figure 7 is a cross section through a capacitor using the electrode of Figure 6 Figure 8 is a view. similar to that of Figure 3, of a still further form of inner electrode, and

Figure 9 is a cross section through a capacitor using the electrode of Figure 8.

Referring now to Figures 1 to 3 the measuring capacitor here shown comprises an outer tubular electrode It made of conductive material, preferably aluminium, which is secured at one end to a flange I I whereby the capacitor may be mounted on a liquid container. The capacitor also comprises an inner tube I! which is spaced from the outer electrode ill by insulating means and insulating material such as a phenolic resin impregnated paper. The outer surface of tube I2 is coated with a conductive material such as by spraying the tube with aluminium or zinc vaporized in a spray gun according to known methods. Electrical connection with this conductive layer may be effected by means of a terminal member I! which is riveted or otherwise secured to the tube I2 and connected electrically with the conductive layer.

If the conductive layer is applied uniformly all round the tube I2 the capacitor will have a uniform capacitance per unit length. If however the conductive layer is arranged so that it does not cover all the surface of the tube l2, the law relating capacitance and length may be made non-uniform to any desired extent.

lintheformshowninllauresitoiitheconductive layer ll applied to the surface of the tube It is of greatest width at the central portion of the tube and tapers towards the upper and lower ends, being connected at the upper end with terminal I! by a strip of conductive material l5. By a suitable design of the shape of the conductive layer in relation to the shape of the liquid container the indicating device may be calibrated linearly in terms of mass or volume of liquid. More specifically, if the width of the conductive layer at any height along the tube is made proportional liquid in the container at that height then, if the indicator follows a linear law as regards change in capacitance, the indicator may be calibrated to show linearly change in'volume or mass of liquid in the container.

Accordingly when the shape of the container is known, however irregular it may be, a mask may be designed to control the extent to which the surface of the tube is coated with a conduc-. tive layer during a metal spraying process, and any number of electrodes may be constructed all of which in conjunction with an outerelectrode will give a capacitor having the desired law.

As pointed out in the specification of my prior application for Letters Patent Serial No. 66,222, filed December 20, 1948, it is most desirable that for the purpose of standardisation the initial or empty capacitance of measuring capacitors may be adjusted and the construction described in that specification may be adapted for use with the measuring capacitor of this invention.

Referring now to Figures 4 and 5 it will be seen that the developed view of the inner tube |2' shown in Figure 4 is generally the same as that of tube l2 shown in Figure 3 except that there is an uncoated longitudinally extending strip it between two coated portions l1 and I8 which are connected to terminal l3 by coated strips 20 and I9 respectively. As seen in cross section in Fi ure 5 this uncoated strip l6 may be arranged to be situated adjacent an inwardly directed corrugation 2| formed in the outer electrode tube It. By rotating the tube ||l relatively to the tube l2 the corrugation 2| may proach the edge of the coated portion ll or II and the initial capacitance will accordingly be changed uniformly along the length of the capacitor.

In a converse arrangement shown in Figures 6 and 7 the inner tube l2" has the coated strip 22 instead of the uncoated strip It so that relative movement of the corrugation 2| and strip 22 will change the initial capacitance. Strip 22 is spaced from a coated portion 23, and the coated portions 22 and 23 are connected to terminal l3 by coated strips 24 and 25.

If desired the uncoated strip It (or equivalently the coated strip 22) may be arranged to extend spirally round the surface of the inner tube for co-operation with a spirally arranged corrugation in the outer tube. In Figures 8 and 9 the inner tube IT has a spirally-arranged uncoated strip 26 extending between coated portions2l and 28 and co-operating with a spirallyarranged corrugation 2| on the outer tube "I".

be made to ap-' to the surface area of the It will be evident that by a suitable disposition of the coated portions of the inner tube there may be provided a number of longitudinally or spirally extending uncoated strips for-cooperation with suitably disposed corrugations on the outer tube so that the initial capacitance may be adjusted by relative rotation of the tubes without alteration of the desired capacitance-length law.

I claim:

A capacitance type gauge for immersion in an irregularly shaped container for a dielectric liquid to measure variations in the depth of the liquid comprising, in combination, two tubular electrodes of different diameter, means mounting said electrodes in telescoped, normally fixed. radially spaced relation, said mounting means being constructed to permit relative circumferential adjustment of said electrodes, one of said electrodes comprising a tube of insulating material having on a part only of a surface thereof a conductive layer, said conductive layer being irregularly shaped and so related to the shape of the container that the circumferential extent thereof at any given depth of liquid is proportional to the surface area of the liquid at that depth, said conductive layer being formed to define on said insulating tube a non-conducting, longitudinally extending area, the other of said electrodes being deformed in the direction of the length thereof, and over substantially the entire length thereof, to provide as viewed in transverse section, segments of different radius. and means for effecting relative circumferential adjustment of said electrodes for initially adjusting the capacity of said gauge when immersed in said container to a given depth.

STANLEY JAMES SMITH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,108,793 San Martin Aug. 25, 1914 1,595,810 Allcutt Aug. 10, 1926 1,610,122 Edenburg Dec. 7, 1926 1,735,889 Blough Nov. 19, 1929 2,361,274 Cravath et al Oct. 24, 1944 2,377,275 Smith May 29, 1945 2,409,073 Sias Oct. 8, 1946 2,464,716 Piton Mar. 15, 1949 FOREIGN PATENTS Number Country Date 243,318 Great Britain Dec. 20, 1928 

